Smoking remains the leading cause of preventable death in the United State. Prolonged exposure to nicotine produces molecular and cellular neuroadaptations that promote the transition from initial nicotine use to abuse and dependence. At the systems level, the process involves the interaction of neuronal reward circuits that drive drug use with other circuits that act to limit dosing to avoid negative effects or even death. The habenula is an epithalamic structure that regulates negative reward during the addiction process. The Specific Aims of this proposal arise from the hypothesis that nicotine-induced changes in the medial nucleus of the habenula (MHb) evolve as nicotine exposure continues through time. These neuroadaptations contribute to the transition from initial use to abuse, and also mediate behaviors that emerge during nicotine withdrawal and promote relapse. Our goal is to understand how nicotine influences the MHb circuitry in vivo because such knowledge can contribute to the design of more effective strategies for smoking cessation. To record neuronal activity from ensembles of neurons, in the intact circuitry, we have designed an ultralight microdrive that allows, for the first time, long-term tetrode unit recording from the MHb. First, we will employ this system to characterize the activity patterns of the MHb circuit in non-anesthetized, freely moving mice. Mice will be studied at baseline to determine patterns of circuit activity. We will also examine neuronal activity in mice undergoing a behavioral task that engages the MHb circuitry to follow natural contributions of the circuitry. Finally we will study how the circuit responds to optogenetic manipulations of local cholinergic activity. That initial experimentation lays the foundation for our studies of nicotine. In those experiments, we will follow neuroadaptations in the MHb circuitry as nicotine administration progresses from acute to chronic, mirroring the progression experienced by smokers. This unique combination of pharmacological, behavioral, and genetic manipulations while we record with in vivo tetrode arrays will provide valuable insight into the circuit-based neuroadaptations that underlie the transition from acute nicotine use to nicotine addiction.